ATLAS Experiment, CERN, Switzerland

Experiment Atlas

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ATLAS is a particle physics experiment at the Large Hadron Collider at CERN that is searching for new discoveries in the head-on collisions of protons of extraordinarily high energy. ATLAS studies the basic forces that shape our Universe since the beginning of time.

Scientists and students from the Institute of Particle and Nuclear Physics are participating in analyses of a wide variety of physics processes and also contribute to a smooth operation of the experiment. Our field of study includes measurements of Higgs boson production, top and beauty physics, heavy ion physics, forward physics and QCD. We have also contributed to a construction of the detector (inner detector, hadronic calorimeter, and forward detector Alfa) and will take part in the planned upgrade of the detector.

People

Higgs boson

The higgs boson decay candidate in the ATLAS Experiment

The higgs boson decay candidate in the ATLAS Experiment

Higgs boson is an elementary particle which is required in the Standard Model theory to explain the origin of particles’ mass. It was observed for the first time at LHC (Large Hadron Collider) at CERN in 2012. Francois Englert and Peter Higgs were awarded by the Nobel prize for the Higgs boson prediction one year after the experimental discovery.

The Higgs boson is very unstable and decays immediately after its production. That is why only its decay products can be measured in the ATLAS detector. There are multiple possibilities how the Higgs boson can decay. One of the decay modes is H →τ + τ which is the channel we study at IPNP. The measurement in this channel is complicated due to the presence of 2-4 neutrinos in the final state which cannot be measured with the ATLAS detector. However, the channel is important because it represents a direct coupling of the Higgs boson to fermions. Our group is involved in the H→ τ + τ analysis in the subchannel where both tau-leptons decay leptonically (τ → e/μ + νe/μ + ντ). In the Run 1, we participated in the data analysis [ATLAS-CONF-2014-061, paper]. Currently, our main interests are the Higgs boson mass reconstruction and the preparation of the common analysis code for the forthcoming Run 2.

B-physics

Measurement of the CP violating phase in the Bs decay

Measurement of the CP violating phase in the Bs decay

B-physics studies hadrons which contain b (bottom) quark, second heaviest of the six quarks. Quarks, together with leptons, form fundamental building blocks of all known matter in the Universe. The b-quark cannot be found in the nature around us. Powerful colliders such as the LHC are needed to create hadrons containing b-quark. Furthermore, these B-hadrons decay rapidly into less exotic and more stable particles. Mean lifetime of B hadrons is measured in picosecond; time so short that particles flying almost at the speed of life travel only few millimeters before decaying. The ATLAS is capable to detect these decays only due to precise tracking detectors, which were developed with the help of people from our department.

Search for physics beyond the Standard Model

Standard model of particles and interactions is very successful theory, which has so far managed to satisfactorily explain all the observed phenomena in the world of elementary particles. Despite its success (or because of it) physicists are trying to find its weak points. Such a discovery would open doors to new development in the field and would help theorists to better understand the universe and formulate new, more complete, theories. Testing of the Standard model and search for the new physics is one of the main goals of the B-physics. This is done for example by studying decays of Bs mesons to pairs of muons or by measurement of the CP violating phase in its decay Bs->Jpsi+phi. So far, experiments ATLAS, CMS and LHCb has not observed any deviation from expectations from the Standard model. However, planned operation of the LHC in next decade promises significant increase in collected data statistics, which will lead in great improvement of the current experimental precision and will enable future measurements.

QCD and Jet Physics

Jet spectrum measured by ATLAS

Jet spectrum measured by ATLAS

Jets are collimated bursts of many particles flying to a narrow cone. At the LHC, they are born in collisions of high-energetic protons. Jets are detectable traces of quarks and gluons – the proton constituents. Quarks and gluons cannot be observed directly. Therefore, jets are used to study their properties and interactions. Precise jet reconstruction on ATLAS can lead to the confirmation of the current theory in the region of the highest energies ever reached. It can also lead to better knowledge of the proton structure or even to a discovery of completely new physical theories. The figure represents a jet spectrum measured by ATLAS. The spectrum can be interpreted as a probability to produce a jet with given direction and transverse momentum in the proton-proton collisions.

Tile Calorimeter

Calorimeters in the particle physics are experimental devices designed to measure energies and directions of the incoming particles, both charged and neutral. The incoming primary high­energy particle interacts with the calorimeter material and produces secondary particles, which may also produce further particles etc, rapidly decreasing in energy. Thus, the primary particle initiates a shower of secondary particles. The shower is finally absorbed in the calorimeter. The signal from secondary particles is measured in the active calorimeter parts and its total sum is proportinal to the total energy of the primary particle. There are several calorimeters in the ATLAS detector. The Tile Calorimeter (TileCal) is the central hadronic calorimeter. It is built of alternating layers of absorber (iron) and sensitive material (plastic scintillator). Charged secondary particles produce scintillating light signal that is measured by photomultipliers. TileCal is equipped with 3 different calibration systems that monitor the signal propagation at various stages. The associated calibration constants are used to convert the measured signal to the particles’ energies. The scientists from the IPNP started working in the TileCal community even before the whole calorimeter was built. Our group was heavily involved in construction, instrumentation and test beam measurements where the performance of the detector was tested. We also participated in the calorimeter commissioning before the LHC turn­on. Nowadays, when the whole detector is built and operational, our main activities are the time calibration and the data quality assessment in the physics data taking as well as the calorimeter Monte Carlo simulations. In near future, the upgrade of the calorimeter electronics is foreseen. Further test beams are planned to check the new electronics and re­evaluate the calorimeter performance. IPNP plans to contribute to these tasks.

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